FIG. 12 and FIGS. 13(a)–13(b) show a conventional wheel of the two-piece type. FIG. 12 is a front view of a wheel; wherein the left half represents a state that a center disk 101 is attached on an integral rim 102 by fastener bolts 103 while the right half a state the center disk 101 is detached from the integral rim 102. FIGS. 13(a)–13(b) are vertical sectional views taken parallel with a rotary axis, wherein the integral rim 102 integrally comprises an outer rim flange 104, a bead seat 106 on an outer rim side, a well part 108, a bead seat 107 on an inner rim side, and an inner rim flange 105. Meanwhile, the center disk 101 is fixed to the integral rim 102 by tightening a plurality of fastener bolts 103 inserted in tapped holes 110 provided in thick-wall portions 109 of the integral rim 102; this thick-wall portions 109 being disposed throughout along a whole circumference of the rim. FIG. 13(b) shows a magnifying sectional view of the thick-wall portion in part encircled by the one-dot-chain line. The fastener bolt 103 usually has a diameter of 6–8 mm in its thread portion, though depending upon the number of bolts used. The thick-wall portion 109 of the integral rim 102, having a tapped hole in size meeting the thread diameter, has a diametrical thickness T of nearly 14 mm for the forged rim of aluminum alloy and requires nearly 16 mm for the cast rim of that material. This however poses a cause of increasing the weight of wheel to a conspicuous extent. Because this thick-wall portion 109 is provided throughout whole circumference of the rim, providing of the thick-wall portion 109 increases a weight of the rim of 18″ diameter (18″ in bead-seat diameter) by about 700 g, for example, even when the rim is a light-alloy forged one. Increase of the weight corresponds to a bulge or an extra thickness exceeding an originally required rim thickness. Even where the integral rim and the disk are formed of aluminum alloy, a two-piece wheel having a diameter 18″ and width 8″, similar with the embodiment of the invention, may have a weight of 10 kg odd, though the weight may be varied with design of the center disk. Such a built-up type is superior in design aspect to the one-piece wheel made with an integral center disk and rim, but has drawback to weight increase. Such weight increase in a built-up type wheel is problematic where considering the fact that the wheels of 19″ and 20″ diameters recently have brought into a practical stage of use. Meanwhile, the thickness of the well part 108 is about 3.2 mm for manufacture with aluminum-alloy forging and is about 5.0 mm for manufacture with aluminum-alloy casting; and thus, as shown in FIG. 13(b), the thickness of the well part 108 is greatly different with that of the thick-wall portion 109. Consequently, stress concentration unavoidably occurs in the boundary portion 111 even if formed with moderate curvature radiuses R, R′ as shown in the figure. Thus, when the wheels are subjected to radial-load endurance tests until fracture of the wheels, there have been many cases of crack occurrence in the boundary portion 111 as a neck portion.
The present invention has been made in view of the above; and is to achieve reducing of the weight in the built-up light-alloy wheels in diameter of 17″, 18″, 19″ or 20″ for four-wheeled vehicles, which are rather big in diameter for the four-wheeled vehicles; and those in greater diameter for two-wheeled vehicles. The present invention is also to achieve improvement in design of the wheel.